Prior work suggests that peer influence of adolescent health risk behavior is a remarkably pervasive phenomenon, but little is known about theory-based motivations underlying peer influence, or moderators that may affect adolescents' susceptibility or resistance to peer influence (DHHS, 1994). This application offers a theoretical model of peer influence and a series of two experimental studies that will examine this model and its implications for potential prevention efforts. Notably, these studies offer an opportunity for translational research, integrating theories and methods from developmental psychopathology and social psychology to elucidate directions for future preventive interventions. The proposed model suggests that adolescents' conformity to peers is motivated largely by a desire to achieve high levels of peer status, and consequently a favorable self-concept. Proposed studies each include an experimental and a longitudinal component to examine mechanisms and moderators of peer influence, as well as a potential preventive intervention to reduce peer conformity. Experimental studies use a simulated chat room context in which electronic confederates ostensibly communicate social norms endorsing risk or prosocial attitudes. Experimental studies allow for an examination of adolescents' public conformity and private acceptance of health risk behaviors (as well one measure of actual aggressive behavior measured in vivo). These studies also allow for the study of mechanisms (e.g., changes in self- esteem or perceived peer status) in real time, as conformity occurs. Longitudinal components to these studies allow for a long-term examination of peer influence susceptibility (Study 1) and the effects of a theoretically-based preventive intervention that may mitigate peer influence effects (Study 2) during the critical developmental interval associated with sharp increases in adolescents' health risk behavior engagement. PUBLIC HEALTH RELEVANCE: Peer influence contributes substantially to adolescents' engagement in health risk behaviors (e.g., substance use). The proposed studies will help determine how and why adolescents are influenced by peers, why some adolescents are more susceptible to peer influence than others, and how prevention efforts may be able to mitigate peer influence effects.